Interval I-Frame v sistemih varnostnih kamer

I-Frame in I-Frame Interval sta izraza, ki povzročata zmedo pri nastavitvah sistema varnostnih kamer. Kaj so ti izrazi in kaj pomenijo v kontekstu CCTV?

Da bi razumeli te izraze, morate vedeti, kako so ustvarjeni video okvirji. Na osnovni ravni je video posnetek sestavljen iz niza sličic. Običajno označen kot FPS (sličice na sekundo) je vsak okvir mirujoča slika, ki ob zaporednem predvajanju ustvari gibljivo sliko.

Videoposnetek s 30 sličicami na sekundo pomeni, da se za vsako sekundo videoposnetka predvaja 30 »slik«. Ko se te slike/okvirji predvajajo zaporedno in hitro, ustvarijo tako imenovani video.

Če vzamemo 30 zaporednih sličic in jih razširimo, bodo videli veliko elementov, ki so precej enaki.

Vzemimo primer, nekdo govori, medtem ko stoji pred zidom. Malo verjetno je, da se bodo informacije okvirja glede te stene spremenile. Posledično bo veliko od teh 30 sličic izgubljena pasovna širina, uporabljena za prenos informacij, da se nekaj sploh ni spremenilo (v našem primeru stena).

Očitno je bila razvita tehnika stiskanja videa, ki razdeli okvirje v bloke in nato išče redundance med bloki. Torej, če se stena v našem primeru ne bo spremenila, zakaj ne bi še naprej uporabljali istih blokov v naslednjih okvirih, da zmanjšate prostor ali pasovno širino? Tu prideta v poštev tako imenovani I-frame in interval I-frame.

Kaj je I-Frame v sistemih varnostnih kamer?

I-okvir je celoten okvir slike v videu in je kodiran brez sklicevanja na druge okvire. Naslednji okvirji (imenovani delta okvirji) vsebujejo samo informacije, ki so bile spremenjene.

Stiskanje videoposnetka se doseže s primerjavo okvirjev, ki sledijo okvirju I (ključni okvir) in pošiljanjem sprememb samo do naslednjega generiranega okvirja I. Številni varnostni sistemi omogočajo uporabniku, da izbere, kako pogosto se generira ta I-okvir.

Z drugimi besedami, I-okvir je okvir, ki se uporablja kot primarna referenčna točka, ki se primerja z drugimi okvirjev v toku. Ta tehnika je namenjena zmanjšanju pasovne širine tako, da samo enkrat pošlje celotno začetno sliko (I-okvir), nato pa delne okvirje (imenovane p-okvirje), ki vsebujejo samo spremembe prizora od začetnega I-okvira.

This method allows for frames that are much smaller in size because they are only pulling the changes in reference to the original I-frame. By doing this, the system can save space and bandwidth.

What is I-Frame Interval in Security CCTV Systems?

The number of interval frames between two I-frames. As we explained above, I-frames (also called keyframes) are used to record information about the entire image. The larger the value of the I-frame interval, the smaller the storage space occupied by the compressed video.

When you select H.264+ or H.265+ on the camera’s encoding setting, the I-frame interval setting is greyed out because it is set and managed automatically and you can’t change it. In other codecs such as H.264H or H.265, you can actually modify the value of the I-frame interval.

Most manufacturers today will allow you to control the I-frame interval which by the way it’s named differently depending on the manufacturer. By default, most security systems use 1 I-frame every 30 images. You can lower this down and there are some tradeoffs in doing so. Keep in mind these tips when modifying the I-frame or the I-frame interval of your camera system:

• Increasing the number of I-frames improves the video quality, but uses more bandwidth. You’ll have fewer recorded days on your camera system.
• The lower the number on the I-frame interval, the smaller the interval between I-frames (the keyframes), and the better the overall quality. But, it will use more bandwidth and reduce and take more space on the hard drive.

Most of the time there’s no need to change the default value of the I-frame and the I-frame interval. However, if you think that your picture quality is not good enough, then you can access the encoding settings and play with these settings until you’re happy with the result. Below we’ve listed the optimized I-frame settings for a few manufacturers.

Recommended I-frame interval settings for Hikvision and their OEMs. The I-frame interval value is recommended to be 50. However, you can try 30 or even 60 and see if you’re okay with the picture.

Recommended I-frame interval settings for Dahua and their OEMs is 60. You can try higher as well to check if you notice any difference. Most likely, there’s no need to modify the default values.
Recommended I-frame interval settings for Axis camera systems. In this case, they call it GOP length which stands for “group of pictures”. Recommended I-frame interval settings for Foscam cameras and their systems. Here it is called “Key Frame Interval”. As we mentioned, the term varies between manufacturers.

The I-frame interval values recommended above offer a stable stream and a good picture quality. However, you can modify them accordingly to match your situation. Keep in mind, the more I-frames and the shorter the I-frame intervals the more bandwidth will be used and less space on the hard drive. You may need to keep it balanced.

From our experience, it’s okay if you use the default values that the camera comes with. The difference varies on the light conditions and sometimes can be minuscule. If necessary, increase the I-frame value only on cameras that are deemed important on your setup.